The present invention relates to a method for making steel from iron oxide-containing raw materials and more particularly to a method in which the raw materials are initially converted by direct reduction with a reducing gas to substantially metallized sponge iron which is then melted in a melting vessel filled with a metal melt.
It is known that sponge iron can be produced by reducing fine-grained iron ores in a fluidized bed reactor with gaseous reduction agents, such as, for example, carbon monoxide and hydrogen. The metallic iron contained in the so-produced fine-grained sponge iron is easily reoxidized. Therefore, the fine-grained sponge iron must be protected against reoxidation before it is melted (which melting is usually a discontinuous process) by changing it into a solid form by means of so-called hot briquetting or by storing and transporting it in inert gas.
In order to be able to effect the melting of the sponge iron at a justifiable cost, the sponge iron must contain as large an amount as possible of metallic iron. The high degree of metallization of the sponge iron desired in the reduction of the ore produces high costs, however, and requires particularly effective protective measures to prevent reoxidation.
It is also known to melt sponge iron by adding electrical energy or combustion heat in a suitable apparatus. Natural gas, crude oil and coal can be used to produce the combustion heat. Suitable melting devices are hearth furnaces (electric arc furnaces, Siemens-Martin furnaces), shaft furnaces (blast furnaces, cupola furnaces, electric low-shaft furnaces) and crucibles (oxygen refining converters) in which in addition to the melting process, alloying, final reduction and/or refining processes can also take place. Thus, it is possible, for example, to melt sponge iron in a blast furnace and at the same time subject it to a final reduction, the end product being pig iron which is rich in carbon. When pig iron is refined in a converter, the carbon contained in the pig iron to an amount up to 4% is burned by the blown-in air or oxygen and the heat produced by this process can be utilized to melt the sponge iron. The capacity of the converter for sponge iron to be melted is undesirably limited, however, by the carbon content of the pig iron in the converter. Further, the nozzles with which the oxygen is introduced into the liquid pig iron are subject to heavy mechanical, thermal and chemical stresses which lead to malfunctions in the refining and melting processes. Many attempts have therefore been made to introduce larger quantities of heat into the converter by suitable measures and to reduce the stresses inherent in the process to which the oxygen injection devices are subjected. In one such prior art method, the metal bath in the converter has been heated by an oil heating system operated with oxygen, but this method has not found acceptance because the capacity of the converter for sponge iron to be melted could not be substantially increased in view of economical considerations due to insufficient heat transfer from the combustion gases to the metal bath.
It is also the custom to introduce the sponge iron to be melted into the converter in charges and to remove the molten steel present after the refining process in a discontinuous manner so that longer starting and dead times are encountered for the converter.
It is also known to melt sponge iron by introducing an oxygen-containing gas and carbonaceous material, preferably solid carbonaceous dust, such as coal dust or coke dust, into a melting vessel containing a metal bath. In this process, the oxygen-containing gas and carbonaceous material preferably are blown into the metal melt below the bath surface and take part in a reaction with each other which produces heat and a carbon monoxide-containing exhaust gas. In such a process, this heat can be utilized in part for melting the sponge iron in the melting vessel and the exhaust gas can be utilized in part for the direct reduction of the iron ore and in part in some other way, or can be utilized entirely in some other way. The melt can then be conducted into a further vessel in which it is processed to steel by suitable metallurgical measures.
An improved process for making steel from fine-grained iron ores is described in U.S. Pat. Application Ser. No. 540,957, filed on Jan. 14, 1975, in the name of Rolf Wetzel et al, assigned to the same assignee as the present application, and incorporated herein by reference. The process described in U.S. Patent Application Ser. No. 540,957 includes the steps of (a) preheating and partially reducing the fine-grained iron ores in a countercurrent heat exchanger; (b) reducing the preheated and partially reduced fine-grained iron ores to fine-grained sponge iron, preferably at 500.degree. to 850.degree. C, with exhaust gas which consists essentially of carbon monoxide and which emanates from a melting vessel in which sponge iron is melted; (c) separating the fine-grained sponge iron formed in the fluidized bed reactor from reduction gas which forms during the reduction in the fluidized bed reactor and pneumatically transporting the separated sponge iron into a carbon-containing metal melt in the melting vessel; (d) melting and finally reducing the sponge iron in the metal melt with the carbon in the metal melt and with further carbonaceous material, preferably coal dust, which is pneumatically introduced into the metal melt; (e) refining the metal melt with pure oxygen to which lime and further additives may have been added; and (f) introducing the oxygen, the carbonaceous material, and the sponge iron into the metal melt through at least one nozzle which is disposed below the surface of the metal melt. In the process described in Patent Application Ser. No. 540,957, the exhaust gas from the melting process is divided into a plurality of streams some of which are then used in various parts of the process, such as, for example, as carrier streams for introducing the sponge iron into the metal melt and as reducing gas. In addition, a preliminary steel is taken from the melting vessel and is refined to steel in a further vessel.